In the manufacture of continuously-extruded products, it is sometimes desirable to extrude a layer of polymer onto an outer surface of a product. In some instances, it is beneficial to tightly bond this additional layer of polymer to the product.
For example, in the manufacture of triple wall pipe, such as corrugated polyethylene or polypropylene pipe, an outer layer of polymer may be extruded by a cross-head die onto the outside of a double wall pipe, as described in U.S. patent application Ser. Nos. 11/078,323 and 12/019,738 to Goddard et al. In order to improve the strength and durability of the resulting triple wall pipe, it may be desirable to improve the bond between the extruded outer layer and the underlying corrugated layer of the double wall pipe. One method for improving the bond includes extruding the outer layer onto the corrugated layer before the corrugated layer has fully cooled from its own extrusion process. Such a method may ensure that the two polymer layers intersperse and bond while at least partially molten and then cool into a uniformly-joined polymer structure. Because this method is not always sufficient, additional techniques may be desired for improving the strength of the bonding between the two layers.
For this reason, various attempts have been made at urging an outer layer of molten polymer firmly against a product as it is extruded. One method for achieving this includes lowering the atmospheric pressure in the space between the molten polymer and the product passing through the extrusion die. In the past for example, a fixed, upstream component of the extrusion die has been shaped such that it closely abuts, and even compresses, the extruded product. This upstream component of the extrusion die might have a fixed inner diameter that is equal to or less than the outer diameter of the extruded product, so as to create a permanent seal between the extrusion die and the product. For example, fixed-diameter sealing rings have been disposed at an upstream end of the extrusion die in order to form a closed space between the extrusion die and the product. A vacuum is then applied to the closed space to urge the molten polymer layer against the product. Unfortunately, such attempts often result in an unacceptable level of friction, or drag, against the travel of the product through the extrusion die.
Such fixed, drag-inducing extrusion die components may be particularly problematic when the product, such as a corrugated pipe, includes sections designed to have increased or decreased outer diameters. Moreover, during the extrusion of triple wall pipe, such drag may be especially undesirable during start-up or shut-down of the pipe extrusion process, when a puller is unable to move the product against the friction of the fixed sealing rings. Thus, fixed extrusion die components are often unsuitable for reliably sealing the product as it passes through the extrusion die.
Accordingly, there is a need for a vacuum seal and related methods for selectively forming a vacuum between an extrusion die and a continuously-extruded product.